This invention relates to a drive unit for a brushless fan motor and control method therof.
A brushless fan motor generally includes a stator provided thereon with a plurality of excitation windings and a rotor including a plurality of rotor magnetic poles each constituted by a permanent magnet. The brushless fan motor thus generally constructed is driven and controlled by a drive unit. A drive unit which has been conventionally used for this purpose generally includes a position detector for detecting a position of the rotor, a rotational speed detecting means for detecting a rotational speed of the rotor, a plurality of excitation changing-over semiconductor switches each connected in series to each of the excitation windings so as to permit an excitation current to flow therethrough to the excitation winding when they are turned on, and a drive circuit for outputting an on/off change-over signal for each of the excitation changing-over semiconductor switches depending on an output of the position detector, a power feed semiconductor switch arranged between a plurality of excitation windings and a power supply so as to permit power to be fed from the power supply to exciting windings when it is turned on, and a power control circuit for controlling the rotational speed of the rotor by controlling the on/off operation of the power feed semiconductor switch. The conventional drive unit using PWM control generally includes a power control circuit provided with a triangle wave generating portion (triangle wave having a constant cycle). The slice level of the triangle wave is changed based on the value of the target rotational speed for the rotor given as the speed command to the power controlling circuit and the value of the actual rotational speed detected by the rotational speed detecting means. The change in the slice level of the triangle wave controls the pulse width of the signal controlling the on/off operation of the power feed semiconductor switches. The power fed to the excitation windings is varied by the on/off of the power feed semiconductor switches, thereby the speed of the brushless fan motor is controlled.
The conventional circuit controlling PWM is a complicated circuit and when such a control is executed with a micro computer, an expensive micro computer is necessary.
It is an object of the present invention to provide a drive unit for a brushless fan motor which is capable of easily controlling the speed of the fan motor without using PWM control.
The present invention has been made in view of the foregoing disadvantage of the prior art.
In accordance with the present invention, a drive unit for a brushless motor which includes a stator provided thereon with a plurality of excitation windings and a rotor including a plurality of rotor magnetic poles each constituted by a permanent magnet is provided. The drive unit generally includes a position detector for detecting a position of the rotor of the brushless fan motor, a rotational speed detecting means for detecting a rotational speed of the rotor, a plurality of excitation change-over semiconductor switches each connected in series to each of the excitation windings so as to permit an excitation current to flow therethrough to each of the excitation windings when it is turned on, and a drive circuit for outputting an on/off change-over signal for the excitation changing-over semiconductor switches depending on an output of the position detector. The drive unit further includes a power feed semiconductor switch arranged between a plurality of excitation windings and a power supply so as to feed power from power supply therethrough to excitation windings when it is turned on, a power control circuit for controlling the rotational speed of the rotor by controlling the on/off operation of the power feed semiconductor switch. The power control circuit controls the on/off operation of the power feed semiconductor switch based on the value of the target rotational speed given as a speed command and the value of the actual rotational speed obtained by the speed detection means.
In the present invention, the power control circuit is constructed in such a manner that after the rotational speed of the rotor is stabilized, the actual rotational speed of the rotor is compared with the target, rotational speed of the rotor. When the actual rotational speed of the rotor is slower than the target rotational speed of the rotor, the turn-off time of the power feed semiconductor switch is shortened. When the actual rotational speed of the rotor is faster than the target rotational speed of the rotor, the turn-off time of the power feeding semiconductor switch is lengthened, and when the actual rotational speed of the rotor is substantially equal to the target rotational speed of the rotor, the turn-off time of the power feeding semiconductor switch is kept at the same level as it is. Such a construction permits power fed from the power supply to excitation windings to be controlled and as a result the rotational speed of the brushless fan motor is controlled. When the turn-off time of the power feed semiconductor switch is varied, as in the present invention, according to the actual rotational speed of the rotor with respect to the target rotational speed of the rotor, the rotational speed of the rotor of the brushless fan motor can be easily controlled just as the power feed semiconductor switch is controlled under PWM control. If the change is only in turn-off time, such a change can be easily realized in either case of hardware power circuit or software power circuit, thus cost for such device can be reduced compared with before.
Just as rotational speed of the rotor can be controlled by changing the turn-off time of the power feed semiconductor switch, the rotational speed of the rotor can be controlled by changing the turn-on time of the power feed semiconductor switch.
Generally the hall device for detecting magnetic flux of a plurality of permanent magnets of the rotor is provided on the side of the stator. In the present invention, the position detector and the rotational speed detecting means may be constructed so as to detect the position of the rotor and the rotational speed of the rotor based on the output by the hall device.
It is possible that the rotation of the rotor is not stabilized and the rotational speed may not increase when the turn-off and turn-on time of the power feed semiconductor switch is changed from during the starting-up operation of the brushless fan motor. From such a view point, it is preferable that the turn-on time and the turn-off time of the power feed semiconductor switch in the power control circuit is set at a predetermined time respectively until the rotational speed of the rotor is stabilized.
For example, the rotation of a fan motor being stopped means a computer having the fan motor equipped therein being stopped, when a brushless fan motor is used for cooling a computer. Therefore, it is preferable that such a brushless fan motor is arranged so as not to stop as much as possible. Accordingly a power control circuit may be set in such a manner that the target rotational speed is normally set at a level (normal rotational speed) which is slower than a maximum rotational speed, and at a time point when a speed command is not detected, while power is being fed from the power supply, turn-off time is then set at zero (0) and the rotor is rotated at the maximum speed. Such an arrangement permits a brushless fan motor rotate with changeable rotational speed in a normal condition and is arranged to rotate at the maximum speed to prevent the temperature of the apparatus to be cooled from rising in an abnormal condition such as no speed command being detected.
Also it is preferable that the power feed semiconductor switch is turned off or an alarm is given when the rotational speed of a rotor does not reach or surpass the predetermined level after a speed command is entered while the power is being fed. Such an arrangement prevents excitation windings from being burned down in case of a failure in starting-up operation between the time points of starting power feeding and the rotational speed""s reaching the predetermined level.
Furthermore, in the present Invention, a plurality of brushless fan motors are rotated at a normal speed which is slower than a maximum speed, and when at least one or more of the plural brushless fan motors are stopped, the remaining brushless fan motors are rotated at the maximum speed. Such an arrangement can prevent an overheating of electronic apparatus under an abnormal condition, in which, for example, at least one or more of the brushless fan motors are holted, by arranging the remaining brushless fan motors to rotate at a maximum speed and to blow enough air compasating the function of the holted brushless fan motors.